The SMR process is mainly based on the reforming reaction of light hydrocarbons such as methane that yields to a mixture of hydrogen (H2) and carbon monoxide (CO) in the presence of water vapor. The reaction is endothermic and slow and requires additional heat input, as well as a catalyst to occur. Usually, SMR reactor performances are limited by the heat transfer and not by the kinetic of the reactions.
In industrial practice, the SMR reactor usually comprises tubes placed in a furnace, said tubes being filled with catalyst—usually in the form of pellets—and fed with the process gas mixture of methane and steam.
Several well-proven configurations are available for furnace design as illustrated by FIG. 1 which presents top fired (also known as down fired), bottom fired (also known as up fired), side fired, and terrace wall.
The top-fired technology is one of the most referenced designs and it is proposed by several technology providers. Top-fired furnaces are typically made of a refractory lined firebox containing several rows of catalyst containing tubes. The necessary heat for the endothermic reaction to occur is provided by roof burners placed in rows between the tubes, and also by rows of additional roof burners at the furnace side, along the walls of the furnace. The combustion products out of the burners are usually blown vertically downwards, so that the tube rows face the flames in their upper part. A flue gases exhaust collector is usually provided at the furnace floor level.
The bottom fired technology is less common in modern plants. According to the bottom fired technology, the burners are arranged in row on the floor of the firing area between the tube rows and fire vertically upwards. This type of reformer has an almost constant heat flux profile along the tube.
The main objective of the furnace design (also called firebox design) is to maximize the heat transferred from the burners to the tubes—from the burner flames and also from the walls and the hot flue gas while respecting a tube maximal operating temperature constraint. The tube maximal operating temperature or MOT (also known as maximal operating constraint or MOT) is a function of several factors, and particularly of the tube mechanical load (mainly feed gas pressure), of the mechanical properties of the alloys used for the tubes and of the desired lifetime of the tubes exposed to creep and thermal aging.
Any intensification of the heat transferred to the tubes has a direct positive impact, either by increasing the productivity or by improving the compactness of the firebox which is valuable in terms of capital expenditures. However, intensification of the heat transferred usually implies higher tube skin temperature levels that reduce tube lifetime or require more resistant alloys, which are much more expensive.
Lack of homogeneity in the heat duty distribution in the furnace will lead some of the tubes to be hotter than other, that's why temperature profiles of tubes are critical elements for the furnace design and during operation. Tube temperature profiles provide decisive information when looking for good compromise between performance and durability; a good compromise is actually essential.
During operations, the performances of the furnace are therefore limited by the temperature of the hottest tube; it should not be hotter than the MOT. In the meantime, the process performance, i.e. the productivity or efficiency of conversion, depends on the average tubes heat flux and temperatures. Therefore, the smaller the difference between the hottest tube temperature and the average tube temperature; the better the furnace performance.
Seeking for simplicity, most of the explanations that follow are made with regards to a top fired furnace. However, it is to be noted that most of the figures and explanations apply as well to bottom fired furnaces.
In such a furnace, the catalyst tubes are arranged in rows within the furnace. The feed is supplied through the top part of the tubes; the synthesis gas produced containing hydrogen and carbon monoxide as major components, and residuals, is withdrawn at the bottom part of the tubes. Burners are arranged in rows between the tubes rows and between tubes and walls. Resulting flue gases are extracted through exhaust tunnels.
The large number of tubes and burners make it necessary to add roof beams to support the structure and to ensure safety of the furnace; said supports beams divide the rows in several parts (also known as sections or bays).
FIG. 3 presents a top view of a top-fired furnace with eight rows of 54 tubes each being organized in three sections (or bays) of 18 tubes each—and nine rows of 12 burners arranged also in three sections of four burners each, and parallel to the tubes rows. The rows of burners are ended by a wall (wall along Y axis also identified as “end walls”). For all rows of burners, the end burners facing the end wall are identified as “wall end burners”.
The sections are ended either by an end wall or by a symmetry plane separating two adjacent sections. The end burners closest to the symmetry planes are identified as “symmetry end burners”. This division in sections induces dissimilar boundary conditions for the flame jets leading to merging of the jets towards the center of the sections.
In all the description the expression “row of burners” is to be understood as “row of burners parallel to the tube rows”, the direction of the rows being also identified as X axis; rows of burners that are perpendicular to the tube rows (along Y axis) will be explicitly identified when necessary as “row of burners perpendicular to the tube rows” or “row parallel to Y axis”.
In the furnaces concerned by the invention where burners are placed in rows parallel to the tube rows, for each burner, the direction of the flame jet created by the burner is affected by the interaction with nearby coflowing jets and by the presence of wall (if any).
Hereafter, only the flame jets interaction within a row of burners parallel to the tube rows (along X axis) will be treated by this invention. It is to be noted that all the burners of a row parallel to the tube rows are operated at the same power, which is not the case for the burners of a row perpendicular to the tube rows.
Prior art, and in particular U.S. Pat. No. 7,686,611, US 2011/0220847, US 2007/0128091, US 2015/0217250 have already considered the case of flame jets interaction within a row of burners perpendicular to the tube rows (along Y axis), and the problems that are specific to this direction are therefore not considered in this invention.
However, the problem of the jet flames interaction within a row of burners parallel to tubes rows generates also problems that have not been solved by the prior art, and this invention aims at focusing on the behavior of the burners in rows along X-axis and aims at improving it; more specifically the invention aims at bringing a solution to the lack of homogeneity of the tubes heating along X axis.
Considering now the flame jet exiting a wall end burner; it behaves like a jet of fluid: the flame jet overlooking an adjacent jet flame has to spray through an external stream of fluid flowing in the same direction; on the other hand, the jet flame overlooking an end wall has necessarily its local velocity near the end wall equal to zero. These dissimilar boundary conditions induce a flame jet deflection with respect to the jet axis.
In addition, a high number of tubes and/or burners in each row induces geometrical constraints in the furnace that makes it necessary to add support beams to ensure safety of the furnace; said supports therefore take place in voids (or spaces) that divide the rows in several sections periodically repeated. This division induces additional dissimilar boundary conditions that impact the jet flame, leading to velocity variations across the axis of the jet of the symmetry end burners, which are the closest to the said supports.
This means that the jet flames generated by different burners in a row are submitted to different influences depending on their location in the row, and consequently the tubes receive variable amount of heat depending on their position in the row.
To illustrate this phenomenon, numerical simulations have been made using a 3-D Computational Fluid Dynamic (CFD) solver intended for calculation of the heat transfer between the combustion chamber and the tubular catalytic reactors.
With this aim, top-fired SMR furnace “representative bays” are defined; the “representative bays” defined would have to be “representative” of repeated sections (or bays) described above, and also have to take into account the presence of the walls or of the symmetry planes. The modular standard reformer would then be composed of an assembly of a number of representative bays to achieve the desired plant capacity.
Depending on the number of tubes and burners in the rows and/or additional geometrical constraints, different type of “representative bays” can exist with various numbers of burners and tubes. However, it is to be noted that the invention applies to all types of sections in terms of number of burners, or in term of number of tubes or in term of end-type of the section (either end wall or void separating adjacent sections).
Such representative bays are shown on FIG. 4. For sake of simplicity, the explanations that follow are made with regards to a representative bay composed of a subset of eighteen tubes heated by two rows of four burners of same power, with a end wall at one end of the bay and a symmetry plane at the second end of the bay.
FIG. 5a illustrates the jet flame merging effect due to the deflection of the flame jets close to the wall and to the void separating adjacent sections.
The merging of the jet flames towards the middle of the bay induces an inhomogeneous heat transfer to the reforming tubes; the tubes in the middle of the representative bay reach a higher skin-temperature as shown by the 3-D CFD results on FIG. 5b. In the case presented, the difference between the maximum skin temperature value and the minimum skin temperature value within the representative bay reaches 30° C.
There is, therefore, a problem of lack of homogeneity in the heating along a tube row, and certain embodiments of the invention aim at solving this problem of control of heat flux homogeneity in top fired SMR (and bottom fired as well) by limiting the jet flame merging along the tube rows.